1. Field of the Invention
The present invention relates to a recording device and a recording control method, a reproduction device and a reproduction control method, an output device and an output control method, and programs. The present invention particularly relates to a recording device and a recording control method, a reproduction device and a reproduction control method, an output device and an output control method, and programs capable of performing more reliable and appropriate color gamut compression, even when content data is handled by a plurality of devices.
2. Description of the Related Art
In recent years, various types of digital image devices having different color expression regions have been increasing in number. Particularly in image display devices, the expansion of the color gamut is remarkable. The color gamut has been substantially expanded in the development from CRT (Cathode Ray Tube) display to plasma display, LCD (Liquid Crystal Display), and OELD (Organic Electro Luminescence Display). Further, many imaging devices, such as a digital still camera and a video camera, capable of imaging colors outside an sRGB (standard RGB) color gamut have been appearing. Therefore, a method has been sought which smoothly outputs wide color gamut image data of a captured image to devices having different color gamuts, such as a wide color gamut display, while suppressing color mismatch and hue shift occurring in a high-luminance and high-saturation color.
Major color matching methods for solving the issue of color mismatch between devices include the following three methods, for example.
The first method uses an ICC (International Color Consortium) profile. The ICC profile is a file describing the properties of an image device defined by an organization called ICC. Normally, two profiles including an input device ICC profile and an output device ICC profile are used to output one image file. The input device ICC profile is embedded in the image file in many cases. When the image of the file is output to another device via a PC (Personal Computer), an application on the PC supporting the ICC profile performs appropriate color gamut matching by using the output device ICC profile, to thereby output the image. This method is based on the assumption that image processing and conversion are performed with the use of a PC, or that an output device supports the ICC profile.
Further, there is a method using sRGB space as a system for matching colors without taking account of the difference in color gamut between devices. The sRGB is an international standard of color space formulated by the IEC (International Electrotechnical Commission) in October 1998. The color space was set to ensure color reproducibility between different environments, such as between different PC models and different devices such as a display and a printer, and was formulated on the basis of the color gamut of the CRT display. For example, if a digital camera, a PC, a display, and a printer are all compatible with the sRGB, it is possible to match color appearances without any particular processing, when a captured image is displayed on the display or printed out. As compared with a workflow based on the ICC profile, the use of a PC is unnecessary, and the processing is substantially simple and convenient. However, the sRGB is narrower in expressible color range than other color spaces. Thus, it is difficult in the sRGB to express such colors as emerald green, dark cyan, orange, light red, and yellow. Therefore, the sRGB is unsuitable for professional use wherein photographs and graphic designs are specifically handled.
Further, a movement for color matching using the Exif (Exchangeable image file format) standard is taking place mainly in the digital still camera and printer industries. The Exif is a format standard for recording an image file, and was proposed and formulated by the JEITA (Japan Electronics and Information Technology Industries Association). Most digital camera manufacturers use this format, as well as the DCF (Design rule for Camera File system) which is a file system standard. Specifically, color space can be described as the header information of an image file. By reading the header information, therefore, a printer can perform more appropriate color conversion into the printer color gamut. Particularly, with Exif Ver. 2.21, in addition to the sRGB supported by the previous versions, Adobe RGB, which is wider in color gamut than the sRGB and normally used in such business fields as the printing industry, can also be used as supported color space. Accordingly, it is now possible to express emerald green and dark cyan, which are difficult to express in the sRGB of the related art. A workflow of a printing operation using the Exif standard is called Exif Print. If a printer has a function of reading an Exif header, the Exif Print can be performed. Therefore, the use of a PC is unnecessary, and the printing operation can be performed with a memory card directly inserted in the printer. There is another standard PIM (PRINT Image Matching) similar to the Exif.
However, in a method using the ICC profile, for example, the ICC profile is embedded in an image file. Therefore, the size of the image file may be unnecessarily increased. Further, a PC application or an output device creates a CMM (Color Matching Module) for reading the ICC profile and performing appropriate processing. However, the CMM is created by respective companies, each with an idea unique thereto. Therefore, even if the color matching is guaranteed within a color gamut common to the input device and the output device, not much attention is paid to colors outside the color gamut of the devices. Therefore, in a high- or low-luminance and high-saturation color, which tends to be outside the color gamut, a hue shift often occurs due to natural clip.
The natural clip refers to a phenomenon in which, when a color of eternally supplied image data is outside the color gamut of a device, the color is forcibly expressed by a color within the color gamut of the device. For example, if only the R component of a color represented by RGB has a value greater than the maximum value of the R component of the color gamut, the R component of the color is represented by the maximum value of the R component of the color gamut (natural clip). In this case, the RGB balance of the original color is lost due to the natural clip, and the hue is changed. Such a change in hue is referred to as the hue shift. That is, the original color is expressed by a color different therefrom. Therefore, the occurrence of such hue shift is undesirable.
Further, in the method using the sRGB, for example, the assumed color gamut corresponds to the color gamut of a standard PC CRT monitor, and thus is narrow. A workflow based on the sRGB does not include a process of performing color gamut compression between devices. Thus, the natural clip occurs in a color outside the sRGB color gamut, and the hue shift occurs in a high- or low-luminance and high-saturation color. The use of the sRGB, therefore, may prevent appropriate expression of dark cyan and green colors printable by a common printer and red displayable on an LCD, the color gamut of which has been dramatically expanded in recent years.
Further, in the method using the PIM or the Exif, the output device performs processing in accordance with the image header information defined by the PIM or the Exif. Therefore, the processing load on the output device may be increased. Further, the content of the processing relies on the output device. Therefore, the color reproducibility intended by the input device may not be guaranteed.
Further, all of the methods described above are mainly for processing a still image, and thus are not suitable for color matching of a moving image, wherein the processing is performed in real time.
In view of the above, to realize a practical color reproduction technique not relying on a device, methods for performing appropriate color matching between devices have been proposed (see Japanese Unexamined Patent Application Publication Nos. 09-098298 and 07-236069, for example).
For example, according to the method described in Japanese Unexamined Patent Application Publication No. 09-098298 (corresponding to U.S. Pat. No. 5,933,253), the color reproduction region of an input system is divided into four regions on a two-dimensional plane by the use of two straight lines, and color gamut compression is performed with the compression direction changed for each of the regions.
Further, according to the method described in Japanese Unexamined Patent Application Publication No. 07-236069, for example, only the chromaticity coordinates of eight points indicating representative colors of red (R), green (G), blue (B), cyan (C), magenta (M), yellow (Y), black (K), and white (W) are exchanged. The conversion of intermediate colors between the above-listed colors is performed in accordance with the conversion results of the eight representative colors.
In addition to the above-described methods, a variety of other methods have been proposed as the algorithm for the color gamut conversion as described above.